The present invention generally relates to micromachined semiconductor sensors and their fabrication. More particularly, this invention relates to a sensor comprising a capacitive sensing device capable of exhibiting large mechanical sensitivity and integration with circuitry to reduce input parasitics and improve the overall signal-to-noise ratio. The invention also relates to a manufacturing process by which such a sensing device and circuitry associated therewith can be fabricated on separate substrates prior to their integration.
Motion sensors in the form of microelectromechanical system (MEMS), including accelerometers, are widely used in aerospace and automotive safety control systems and navigational systems, consumer goods such as VCR cameras, as well as other mass-volume applications in which both performance and miniaturization is highly desirable and sometimes necessary. Capacitive micro-accelerometers have the combined advantages of high sensitivity, good direct current (dc) response and noise performance, low drift, low temperature sensitivity, low power dissipation, and large readout bandwidth as compared to high sensitivity tunneling and resonant devices. High sensitivity in capacitive accelerometers is attained in part by using a wafer-thick large proof mass. The first generation of these devices used multiple wafer bonding to form the proof mass and electrodes. More recently, capacitive accelerometers have been fabricated using a combined surface-bulk micromachining process to form the device on a single silicon wafer, such that the micromachined accelerometer is advantageously monolithically fabricated with the interface circuit. All of these devices are hybrid packaged with the interface circuitry, and as such are characterized by relatively large parasitics. A large capacitive sensitivity is generally used to compensation for the relatively high electrical noise floor caused by the parasitics. However, the proof mass of a surface micromachined accelerometer is typically small, which results in lower sensitivity and a mechanical noise floor of typically tens to hundreds of micro-g's.
In view of the above, it would be advantageous if a capacitive micro-accelerometer could be fabricated that provides both large mechanical sensitivity and integration with CMOS circuitry to reduce input parasitics and improve the overall signal-to-noise ratio. In addition, it would also be desirable if such a device and its fabrication process could be achieved with reduced manufacturing and packaging cost for use in mass-volume applications.